VIERA, Fla. (AP) - Stephen Strasburg's sinker may still be a work in progress. His other pitches seem to be rounding into shape quite nicely.

After giving up back-to-back singles - both on sinkers - to start the game, the Washington ace struck out six over three innings Thursday night in a 4-all tie with the New York Mets. The game was called after 10 innings.

Strasburg, expected to be the Nationals' opening day starter, already has an overpowering fastball, a wicked curve and a changeup that just doesn't seem fair. A big goal this spring is getting a better feel for his sinker, but he admitted he's still struggling with it.

"I need to get over the idea of throwing it harder will make it better," he said. "I feel like I'm just kind of throwing through the sink and it's kind of flattening out. I want to throw it a little bit slower, let it move a little bit more and I'll get better results."

The 24-year-old right-hander is also working on calming himself down and not getting too pumped up in the first inning, which causes him to rush through his delivery. He struggled with that a little bit in his initial spring outing - he gave up a single and home run to the first two Mets batters - but made the adjustment much quicker Thursday night.

"Being so early, I don't feel like I've had enough mound time for it to just click, and not really think about it," Strasburg said. "I'm still trying to feel it out a little bit. The good thing is I felt good when I came out. That's the bottom line."

While Strasburg gave up a run on three hits while also walking two, the Mets got a look at one of their bright, young arms in the form of 22-year-old right-hander Rafael Montero.

Montero was the team's minor league pitcher of the year last season. He made his first spring start and threw two innings, giving up three hits - all doubles - and one run.

"He's got a good arm," Mets manager Terry Collins said. "I like the fact that he really pounds the strike zone with his stuff. Great poise on the mound. Great demeanor. Pretty impressive."

Top catching prospect Travis d'Arnaud got two hits and scored twice for the Mets. Jordany Valdespin also had two hits, both off Strasburg.

Washington prospect Anthony Rendon, the No. 6 overall pick in the 2011 draft, kept up his strong spring with a double and two RBIs. He is batting .417 with a team-high five RBIs.

Denard Span had his best spring game for the Nationals. He doubled in the first inning, then manufactured a run in the third when he walked, stole second and third and scored on a groundout by Jayson Werth.

Strasburg, who will not be on any kind of innings limit this season, was asked if he is motivated to show people what kind of pitcher he can be over the course of an entire season.

"I don't really care about showing people," he said. "I want to prove it to myself."

Strasburg mentioned that one of the pitchers he watched while growing up in the San Diego area was Jake Peavy of the Padres.

"He was a bulldog out there," Strasburg said. "Even if he had little nicks, little tweaks on (his) body, he was going to go out there and give it everything he has every fifth day."

"He was going to throw 200 innings every year. That's the kind of guy I want to be," he said.

NOTES: Nationals 3B Ryan Zimmerman is expected to make his spring training debut Sunday when he will serve as the designated hitter against St. Louis. ... Nationals LF Bryce Harper may have gotten a day off Thursday, but he'll be back in the lineup Friday against the Braves - and playing center field. Manager Davey Johnson said on the days he chooses to rest Span in CF, Harper could be shifted over. Harper played some center field last season. ... Catcher Kurt Suzuki hit in the No. 2 spot. Johnson said he simply wanted to make sure Suzuki got two at-bats before coming out of the game. ... Mets RHP Zack Wheeler did some motion exercises and felt better, Collins said. Wheeler was scratched from his Wednesday start because of a strained right oblique muscle. ... Collins said it's looking as if LHP Johan Santana will make his spring debut either March 10 or 11.